The present invention relates to a method for thermally bonding contact elements of a flexible film substrate to contact metallizations of an electronic component.
To bond flexible film substrates to an electronic component, for example a chip, use has hitherto normally been made of the thermocompression method, in which a so-called thermode is pressed against contact elements of the substrate with the application of pressure and temperature in order to bond said contact elements to contact metallizations of the chip. In order to prevent damage in this process to the temperature-sensitive plastics-material support layer of the substrate, which layer generally has a decomposition temperature which is in the range of the temperature necessary for the bond, it is necessary in the case of the known method to remove the support layer and, possibly, an adhesive layer bonding the support layer to the contact elements prior to the application of pressure and temperature to the contact elements so that a direct access to the contact elements of the substrate from their rear is possible. The removal of the support layer proves in practice to be very complicated, generally, apertures referred to as xe2x80x9cwindowsxe2x80x9d are etched in the support layer of the substrate for this purpose in a separate method. Substrates prepared in this way can then be used by means of a bonding procedure, referred to as xe2x80x9cinner-lead bondingxe2x80x9d, within the framework of an automated bonding method referred to as xe2x80x9ctape automated bondingxe2x80x9d.
By way of example, reference is made to U.S. Pat. No. 4,970,365 for the current state of development of methods for thermally bonding contact elements of a flexible film substrate to contact elements of an electronic component. This publication discloses a method in which laser radiation is applied to the contact elements of a polyimide film substrate for the purpose of bonding to contact metallizations of a component at the rear. In this process, the application to the exposed contact elements takes place remotely from the thin polyimide support layer of the film substrate.
The object of the present invention is to propose a method which makes possible bonding of the contact elements of flexible film substrates to the contact metallizations of electronic components possible without the need to expose the contact elements in a preceding method.
According to a first solution, the application of laser radiation to the contact elements via an optical fibre is provided in the method according to the invention, a transparent support layer or the absorption of the contact elements being used to transmit the radiation with a laser radiation wavelength approximately match thereto essentially through the support layer and to absorb it in the contact elements. Furthermore, an application of pressure is superimposed, according to the invention, on the application of laser radiation, in such a way that, during the application of laser radiation, the contact elements of the substrate and the contact metallization of the component in the region of an application point of the optical fibre are pressed against one another and temperature is applied by means of the laser radiation. Such components may, for example, be chips, but also generally substrates provided with connecting areas. Two film substrates can also be bonded to one another in the method according to the invention.
This application of pressure prevents the formation of air gaps between the contact elements and the contact metallizations, a reliable thermocoupling thereby being provided between the latter. An inadequate thermocoupling could result in a heat build-up in the region of the contact elements, which would, in turn, result in an undesirable load of temperature on the substrate or on the support layer and, possibly, on an adhesive layer bonding the support layer to the contact elements.
As a result of using radiation energy for applying temperature to the contact elements, the good transparency properties provided in the normal case, for example, with a plastics-material support layer formed from polyimide are utilized to generate the temperature necessary for the thermal bonding only in the bonding region as a result of the good absorption properties of the metallic contact elements.
The use of radiation energy for applying temperature to contact elements for the purpose of thermally bonding the contact elements of a transparent substrate to the contact metallizations of a semiconductor is in principle also disclosed in DD 140 942. The substrate disclosed in DD 140 942 has, however, a relatively thick, rigid substrate body which, because of its material properties, is suitable for transferring the contact pressure, applied via a clamping device, necessary for producing the thermal bond between the contact elements and the contact metallizations. In addition, in the known method for introducing the radiation energy into the contact elements, a complicated optical device is necessary which makes it possible to focus the radiation energy with respect to the position of the contact elements.
In contrast to the substrate material disclosed in DD 140 942, the substrate to which the method according to the invention is applied is a flexible film substrate which can be loaded to a substantially lesser extent both thermally and mechanically just because of its relatively small thickness. In this regard, the method according to the invention has the advantage that a discrete or punctiform exposure of the film substrate in the region of the application point of the optical fibre is provided both in regard to the necessary contact pressure and in regard to the temperature necessary for producing the bond. In addition, complicated focusing optics can be dispensed with in this connection, which substantially simplifies the construction of the equipment necessary for carrying out the method compared with the method disclosed in DD 140 942.
The discrete exposure of a contact point of two contact elements to be bonded by means of an optical fibre which serves to introduce both the pressure and the energy necessary for bonding is indeed disclosed in DE 42 00 492 A1. However, in this known method, the optical fibre is in direct contact with one of the two contact elements to be bonded.
If the fibre end surface of the optical fibre is pressed directly against the support layer of the substrate for the purpose of applying pressure, it is possible to carry out the method with a minimum device complexity.
In addition, there is also the possibility of applying energy by means of an optical fibre or light-guide optics and to use a separate, and therefore independent thereof, contact-pressure device to apply pressure. As a result, it is possible to adapt the design of the contact-pressure device to the actual dimensions of the substrate or of the component which exist in a particular case.
One way of applying pressure is to generate reduced pressure in a contact region between the substrate and the component.
In accordance with a second solution, the substrate is bonded to the component in two phases. According to the invention, ultrasonically inducated mechanical vibrations and pressure are applied, in a first phase, to the support layer in such a way that a support-layer region covering a contact region of a contact element is exposed. In a subsequent second phase, pressure and temperature and/or ultrasonically induced mechanical vibrations are then applied to the contact element which has now been exposed on the rear in the connection region for the purpose of bonding to the associated contact metallization.
This second bonding method according to the invention, which is an alternative to the first method according to the invention, makes also possible a contact between a substrate and a component without a pretreatment, carried out separately from the bonding operation and independently of the latter, of the substrate in a separate method. On the contrary, in accordance with the second solution according to the invention, the first phase, which serves as preparation for the actual contacting, and the second phase, the actual contacting phase, are combined with one another in one operation. In this case, there is chosen for the xe2x80x9cexposurexe2x80x9d of the contact elements in the contact region an application of energy which is essentially characterized by ultrasonically induced vibrations and pressure, that is to say forms of energy which prove to be harmless for the plastics-material support layer since they operate only discretely and do not result in a large-area decomposition of the support layer or in delamination between the contact elements and the support layer as does an application of temperature to the support layer. In the case of this second alternative method according to the invention, the temperature necessary to produce the thermal bond is applied, in accordance with the first alternative method according to the invention, only in the bonding region between the contact elements of the substrate and the contact metallizations of the component.
The abovementioned alternative method according to the invention proves particularly advantageous if the energy is applied by means of a pin-like thermode which is used, during the first phase, to apply ultrasound to the support layer and, during the second phase, to apply temperature and/or ultrasound to the contact element. As a result it is consequently possible to carry out both phases with one and the same tool so that the method proves particularly simple in implementation and also only a correspondingly simply designed device is necessary to carry it out.
In accordance with a further achievement, the substrate is again bonded to the component in two phases, but with the different that, during the first phase, the contact element is not exposed, but ultrasonically induced mechanical vibrations and pressure are applied to the support layer in such a way that a support-layer region covering a contact region of one contact element is compressed and then serves, in a second phase, to transmit the ultrasonically induced mechanical vibrations to the contact element. In this case, use is made of the effect that a support layer, which per se transmits ultrasonic vibrations only inadequately because of its flexibility, become relatively rigid in the state of extreme compression and, consequently, becomes a good conductor of vibrations.